Meadow voles, also known as field mice, are common pests in lawns and gardens and can be identified by the very clean very round open holes that they dig, as well as by above ground trails or runways that they use repeatedly. They are strictly an outside pest, as they do not invade homes or other structures the way house mice do. Their populations are cyclical in nature and typically three or four years may pass where their populations remain low and they are only an occasional pest problem, but about every 3-5 years their populations will experience explosive growth and they can become a significant pest in lawns and gardens.
Voles typically secondarily invade pre-existing tunnels made by moles and gophers rather than dig their own(clearly having worked out that a lot less effort is required). Voles nest underground in the tunnels, but also run around on the surface following well worn trails between holes or from lawns into areas of heavy vegetation cover. In addition to causing significant damage to lawns, they can also damage a wide variety of plants that are used for ground covers, and they also eat bark and can girdle and kill landscape shrubs and even small trees.
Both of these images are from one lawn in the Ames area.